In the electronics industry, manufacturers are constantly designing and building smaller, faster components in order to gain improvements in circuit performance and cost. The smaller these components become, the more susceptible they will be to damage from static electricity. Electrostatic discharge (or ESD) is a phenomenon which destroys components and costs manufacturers millions of dollars each year. Many attempts have been made to control ESD in the prior art, but all of them have been only marginally successful.
To fully understand the significance of the problem, it is necessary to understand the mechanism by which electrostatic discharge occurs and the various prior art methods that have attempted to control it. The prior art solutions have basically attempted to cope with ESD by packaging sensitive electronic components and circuits in various forms of containers, a typical example of which is shown in FIG. 1. Circuit board 10 containing ESD sensitive components 12 is completely encased within packaging material 14 which rests upon grounded or static dissipative surface 16. As body 18 reaches to touch the package 14, a spark discharge 20 to the package can occur. This happens because as the charged body 18 approaches the surface of the package 14, a thin column of air becomes ionized and a spark discharge 20 is initiated. This thin column of highly conductive ionized gas forms exceedingly fast (in a nanosecond or less) and, once formed, it behaves electrically very much like an ordinary small diameter wire. In other words, almost all of the voltage across it is inductive voltage (V=L di/dt). Very quickly, electric charge flows through this ionized column and is deposited in a small area on the surface of the package 14. However, as the charge accumulates on this surface spot, the voltage of the spot rapidly rises and approaches that of the charged body 18.
Once the voltage difference between the surface spot and the body 18 gets small enough (about several hundred volts), the ionized column can no longer support itself and it extinguishes. This whole sequence of events, from initiation to quenching, happens very quickly (approximately a few nanoseconds). The main parameter that limits the speed is the inductance L of the thin column of ionized gas.
What happens on the surface of the package 14 during this short event is strongly dependent on the surface resistivity of the package 14. For a material with a relatively high surface resistivity (10.sup.11 Ohms/square), there are many tiny, faint sparks 20 during a discharge. For each tiny discharge, the point on the surface directly beneath the spark 20 has a very high voltage; however, the voltage just a short distance away may be many hundreds of volts less during the actual spark discharge 20. This is because the high surface resistivity slows down the diffusion of the spark-deposited charge. The point where the spark 20 makes contact with the package 14 rises to several thousand volts, but just a short distance away (e.g. one inch) the voltage may be in the low hundreds or less.
Once the spark 20 extinguishes, the concentrated charge on the surface quickly spreads out through surface conduction and the voltage peak quickly decays. These events are quite analogous to dropping a pebble into a pool of water. A column of water of substantial height forms fight at the center, but the height of the rest of the water quickly drops off as the radius increases. As time passes, the height of the center column and the water immediately around it are quickly restored to the average surface height of the pool (which is now imperceptibly greater due to the volume displaced by the pebble).
Packages with lower surface resistivity can have fatter and more aggressive spark discharges 20 to the surface of package 14; but lower resistivity actually improves ESD protection. Since any discharge 20 to the surface of the package 14 has to charge a larger area, more charge must flow for a longer time to attain the same surface voltage which is developed on the lower resistivity package 14. The longer it takes to charge up this larger area, the more time is available for the charge to bleed away from body 18. The greater the amount of charge removed from the body 18 during the spark 20, the more the voltage of body 18 is reduced. The net result is that, although a larger area of the package rises in voltage, the peak voltage at any point on the surface is less.
There have been three types of packaging 14 developed in the prior art to prevent damage to the electronic components inside the packaging 14 caused by ESD spark discharges. They are metal, insulative and static-dissipative.
Metal packages have been constructed which give near-perfect ESD and electrical overstress (EOS) protection. This is because there is virtually no resistive voltage drop anywhere on the surface of the package 14 and the charge on body 18 is shunted to grounded surface 16, dissipating the voltage potential between body 18 and the contents of the package 14. Furthermore, no flux lines can penetrate the metal at the high frequencies of a spark discharge, so there can be no induced voltage between any two points on the inside of the package (where the sensitive components 12 reside). If there is no voltage differential anywhere inside the package 14, then there can be no harmful voltages to damage the sensitive components 12. While metal packages 14 are the most effective, they are generally impractical because of their expense and weight.
A package 14 made of insulative material (e.g. untreated polyethylene) also gives excellent protection to the enclosed circuit board 10, but only as long as the package 14 remains closed. A charged body 18 which touches the package will not generate a spark 20 because the insulative material will not conduct. Further, polyethylene can sustain several kilovolts per rail of thickness, so there is no potential for breakdown from the outside surface of package 14 to the inside surface. However, a charged person who picks up the package 14 will remain fully charged because the package 14 is not able to shunt any charge to the grounded surface 16 when body 18 touches it. If the package 14 is then opened and body 18 touches circuit board 10, there will be a spark discharge, possibly harming the components 12.
The ESD protection offered by a package 14 made from static dissipative material falls somewhere between the metal and insulative packages discussed hereinabove. A charged body 18 can discharge a spark 20 to static dissipative material. However, even though its surface resistivity (10.sup.5 to 10.sup.11 Ohms/square) is not nearly as low as metal, a properly designed static dissipative package 14 can quickly bleed away the charge on body 18, thereby lessening the chance of subsequent ESD damage.
The problem which these and all other prior art ESD protection schemes have in common is that the sensitive components 12 are only protected so long as they remain in the package 14. However, the circuit board 10 must be removed from the package 14 many times during the manufacturing and testing process, and more importantly, the end user must remove it from the package 14 for installation and then discard the package 14. At this point, there is no longer any protection for the components 12 from ESD.